Aqueous active alkali silicate solution having high molar ratio, method for production thereof and method for use thereof

ABSTRACT

An alkali silicate aqueous solution having intermediate characteristics between water glass and colloidal silica and having a high molar ratio (SiO 2 /(A 2 O+B))(A: alkali metal, B: NH 3 ), a high silicon content and a high anionic activity. The alkali silicate aqueous solution has the properties: (A) the molar ratio of silicon to an alkali (SiO 2 /(A 2 O+B) is in the range of 4 to 30, and (B) the silicon concentration in terms of an oxide (SiO 2  concentration) is in the range of 6.8 to 30% by weight. A process for preparing an alkali silicate aqueous solution comprises dealkalizing a starting alkali silicate aqueous solution by means of an electro-dialysis device, and then optionally concentrating the dealkalized solution by a reverse osmosis membrane method.

This application is a 371 of PCT/JP02/02549 filed 18 Mar. 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an alkali silicate aqueous solutionhaving a high molar ratio (SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)), ahigh silicon content and a high anionic activity, a process forpreparing the solution and uses of the solution.

2. Description of Related Art

An alkali silicate aqueous solution called water glass containsrelatively large amounts of alkali ions in order to maintain thesolution state, and therefore the molar ratio of silicon to an alkali(SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)) is usually less than 4.Although silicic acid ions and alkali ions are contained in thesolution, the negative charge quantity is small, so that the anionicactivity is low and the ζ potential that is an indication of the anionicactivity is in the range of −14 MV or less but more than −40 MV.

On the other hand, primary particles called silicic acid sol orcolloidal silica have neither internal surface area nor crystallineportion, and they are dispersed in an alkaline medium. The alkali reactswith the silica surface to form negative charge on the silica surface,and because the silica particles are negatively charged, they arestabilized by virtue of the repulsion of the negative charge between theparticles. On the surface of the silica colloidal substance, however,many silanol groups (Si—OH) are present in addition to the silicic acidanions which form the negative charge. Hence, the negative chargequantity is small and the ζ potential is in the range of −25 to −38 MV.

By the dealkalization of water glass, a silicic acid sol is obtained,but a stable intermediate between the water glass and the silicic acidsol has not been obtained. The reason is that since the molar ratio israised with the progress of dealkalization, the water glass cannotmaintain the solution state thereof. When the molar ratio becomes 4.2 ormore, precipitation of silica takes place and the water glass cannotmaintain the solution state.

If a high-molar ratio alkali silicate aqueous solution havingsolution-like characteristics such as those of water glass and having ahigh molar ratio and a high SiO₂ concentration similar to the silicicacid sol is obtained, development of various uses can be expected.

That is to say, there has been desired a process to raise the molarratio, the activity and the SiO₂ concentration with retaining thesolution-like characteristics of the water glass.

However, the molar ratio cannot be raised by only concentrating thewater glass through evaporation. For example, if a water glass producthaving the highest molar ratio 4.0 is concentrated to a SiO₂concentration of 30% by weight, the product gels completely.

On the other hand, concentration of colloidal silica by ultrafiltrationis carried out (see U.S. Pat. No. 3,969,266, U.K. Patent No. 1,148,950,Japanese Patent Laid-Open Publication No. 15022/1983, etc.). If thecolloidal silica is particle-grown silica, it can be satisfactorilyconcentrated by the ultrafiltation. In case of the water glass, however,the amount of the low-molecular weight component such as ion is largeand the yield obtained by the ultrafiltration is low. Moreover, becauseof large loss of ions, the anionic activity inherent in the water glassis lost.

It is an object of the present invention to provide an alkali silicateaqueous solution having intermediate characteristics between water glassand colloidal silica and having a high molar ratio (SiO₂/(A₂O+B) (A:alkali metal, B: NH₃)), a high silicon content and a high anionicactivity, a process for preparing the solution and uses of the solution.

SUMMARY OF THE INVENTION

The alkali silicate aqueous solution according to the present inventionhas the following properties:

(A) the molar ratio of silicon to an alkali (SiO₂/(A₂O+B) (A: alkalimetal, B: NH₃)) is in the range of 4 to 30, and

(B) the silicon concentration in terms of an oxide (SiO₂ concentration)is in the range of 6.8 to 30% by weight.

The alkali silicate aqueous solution of the present invention preferablysatisfies, in addition to the above properties (A) and (B), at least oneof the following properties (C) to (F):

(C) the ζ potential is in the range of −40 MV to −80 MV,

(D) the peak area at the chemical shift of −100 to −120 ppm in the²⁹Si-NMR measurement is 1.35 times or more of the peak area of waterglass at the chemical shift of −100 to −120 ppm measured by ²⁹Si-NMRunder the same conditions as in the above measurement and is 1.20 timesor more of the peak area of colloidal silica at the chemical shift of−100 to −120 ppm measured by ²⁹Si-NMR under the same conditions as inthe above measurement,

(E) the transmittance within the wavelength region of 1000 to 200 nmmeasured by an absorptiometry is in the range of 90 to 100%, and

(F) the electric conductivity is in the range of 2.1 to 30 mS/cm.

The first process for preparing an alkali silicate aqueous solutionaccording to the present invention comprises dealkalizing a startingalkali silicate aqueous solution, which has a molar ratio (SiO₂/(A₂O+B)(A: alkali metal, B: NH₃)) of less than 4 and a silicon concentration interms of an oxide (SiO₂ concentration) of 2.0 to 12.0% by weight, bymeans of an electro-dialysis device.

In the first process, it is preferable to concentrate the resultingdealkalized solution by a reverse osmosis membrane method.

The second process for preparing an alkali silicate aqueous solutionaccording to the present invention comprises:

dealkalizing a starting alkali silicate aqueous solution having a molarratio (SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)) of less than 4 by meansof an electro-dialysis device, and

concentrating the dealkalized solution by a reverse osmosis membranemethod.

In the above process, the reverse osmosis is preferably carried outusing an alkali-resistant composite membrane of a fractional molecularweight of 100 to 20000.

In the present invention, after the electro-dialysis and/or the reverseosmosis, the resulting alkali silicate aqueous solution may be furtherbrought into contact with a cation exchange resin.

The alkali silicate aqueous solution of the present invention ispreferably used as a main agent of a ground consolidation agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electro-dialysis device used in thepresent invention.

FIG. 2 is a graph comparing ²⁹Si-NMR spectra of a sodium silicateaqueous solution of the present invention, water glass and colloidalsilica (DuPont SM).

FIG. 3 is a graph comparing results of ultraviolet to visiblespectrophotometry of a sodium silicate aqueous solution of the presentinvention, colloidal silica (DuPont SM) and colloidal silica (DuPontHS-40).

DETAILED DESCRIPTION OF THE INVENTION

The alkali silicate aqueous solution according to the invention hasintermediate characteristics between water glass and colloidal silicaand has a high molar ratio (SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)), ahigh silicon content and a high anionic activity.

That is to say, the alkali silicate aqueous solution is characterized inthat the ratio of the silicon content to the alkali content is higher ascompared with usual water glass. The alkali employable herein is, forexample, lithium, sodium, potassium or ammonium, and most generally usedis sodium.

In the alkali silicate aqueous solution of the invention, the molarratio (A) of silicon to an alkali (SiO₂/(A₂O+B) (A: alkali metal, B:NH₃)) is in the range of 4 to 30, preferably 9 to 26, more preferably 12to 21. When the alkali is lithium, sodium, potassium or the like, themolar ratio is a value calculated in terms of an oxide (A₂O wherein A isan alkali metal), and when the alkali is ammonium, the molar ratio is avalue calculated on the basis of ammonia. The alkali metal and ammoniummay be used in combination. In this specification, the expression(SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)) is sometimes referred to as“molar ratio” simply hereinafter.

If dealkalization of usual water glass proceeds to raise the molar ratio(SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)), silica is precipitated and thesolution state cannot be maintained. With regard to the presentinvention, however, the solution state can be stably maintained. It isconsidered that the presence of such anions as mentioned above greatlycontributes to the stable solution state. When the anionic activity ishigh, the silicic acid anions actively function to form an electricdouble layer and thereby maintain the solution state stably, even if Nathat is a polymerization stopper in the water glass is removed.

In the alkali silicate aqueous solution of the invention, the siliconconcentration (B) in terms of an oxide, i.e., SiO₂ concentration, is inthe range of 6.8 to 30% by weight, preferably 8 to 26% by weight, morepreferably 14 to 22% by weight.

The alkali silicate aqueous solution of the invention has a siliconconcentration of almost the same level as that of a silicic acid sol orcolloidal silica.

The alkali silicate aqueous solution of the invention preferablysatisfies, in addition to the above properties (A) and (B), at least oneof the following properties (C) to (F).

The anionic activity is evaluated by a ζ potential. In the alkalisilicate aqueous solution of the invention, the ζ potential (C) is inthe range of preferably −40 MV to −80 MV, more preferably −50 MV to −80MV, particularly preferably −58 MV to −80 MV.

The ζ potential is a parameter relating to dispersion or flocculation ofparticles. When many particles of the same kind are dispersed in aliquid, these particles have the same electric charges. As the electriccharges become higher, these particles repel each other and are heldstably without being flocculated. If the particles have no electriccharge or if a substance of opposite electric charge is contained, theparticles are flocculated or settle immediately. The electric charges ofthe particles depend also on pH of the solution.

In the alkali silicate aqueous solution of the invention, the ζpotential is negative as described above, and many anionic molecules arecontained, so that the solution has a high anionic activity.

The anionic molecules contained in the alkali silicate aqueous solutionof the invention are extremely small, and even when compared withcolloids such as colloidal silica, they are smaller. Accordingly, evenif anionic particles are present, any behavior such as that of a sol isnot observed in the present invention, and the alkali silicate aqueoussolution can be treated substantially as a solution. This is backed upalso with the transmittance described later.

Although the mode of the presence of the anionic particles is not alwaysclear, it is thought that the particles are present as ultra-fineparticles of nm order having SiO⁻ on their surfaces. Various structuresof the silicic acid anions are known as described below. However, it isthought that, in the alkali silicate aqueous solution of the invention,mono- or bifunctional anions assigned to straight-chain polymers orpolycyclic silicic acid anions are few, and a great number oftrifunctional Q3x, trifunctional Q3y and tetrafunctional Q4 arecontained.

In the usual colloidal silica, few of such anions as described above arepresent, and the ζ potential is in the range of about −25 MV to −38 MV.Although the water glass contains anions, the ζ potential is in therange of about −14 MV to −40 MV because of few polyfunctional anionportions.

The alkali silicate aqueous solution of the invention has a high anionicactivity as described above, and hence development of use application,such as yield improver in the paper making, heat-resistant binder,catalyst, inorganic coating agent, reinforcing agent, anti-slip glossagent, adhesive, material of porous product and insulating material, isexpectable.

(D) The peak area at the chemical shift of −100 to −120 ppm in the²⁹Si-NMR measurement is 1.35 times or more, preferably 1.35 to 2.5 timesof the peak area of water glass at the chemical shift of −100 to −120ppm measured by ²⁹Si-NMR under the same conditions as in the abovemeasurement and is 1.20 times or more, preferably 1.20 to 1.33 times ofthe peak area of colloidal silica at the chemical shift of −100 to −120ppm measured by ²⁹Si-NMR under the same conditions as in the abovemeasurement. It can be seen from this result that, in the alkalisilicate aqueous solution of the invention, mono- or bifunctional anionsassigned to straight-chain polymers or polycyclic silicic acid anionsare few, and a great number of trifunctional Q3x, trifunctional Q3y andtetrafunctional Q4 are contained.

The peak area is determined by correcting a base line and thencalculating from an area enclosed with a vertical axis at −100 ppm, avertical axis at −120 ppm and the spectral curve.

In the alkali silicate aqueous solution of the invention, thetransmittance (E) within the wavelength region of 1000 to 200 nm, asmeasured by an absorptiometry, is in the range of preferably 90 to 100%,more preferably 95 to 100%.

The transmittance of the usual water glass is similar to the above, butthe transmittance of the colloidal silica within the wavelength regionof 200 nm to 380 nm is extremely low and 10 to 0%. It can be seen fromthis result that the alkali silicate aqueous solution of the inventionhas properties close to those of water glass.

Further, the alkali silicate aqueous solution of the invention has anelectric conductivity (F) of preferably 2.1 to 35 mS/cm, more preferably2.1 to 16 mS/cm, particularly preferably 5.0 to 11.0 mS/cm. Because ofsuch a high electric conductivity, the alkali silicate aqueous solutionof the invention is a highly desalted solution and is a stable solutionfree from flocculation caused by silicic acid anions.

As described above, the alkali silicate aqueous solution of theinvention has intermediate characteristics between water glass andcolloidal silica and has a high molar ratio, a high silicon content anda high anionic activity.

Although there is no specific limitation on the process for preparingthe above-mentioned novel alkali silicate aqueous solution, the presentinventor has found that the novel alkali silicate aqueous solution canbe prepared efficiently and stably by the following first and secondprocesses.

The first process for preparing an alkali silicate aqueous solutionaccording to the invention comprises dealkalizing a starting alkalisilicate aqueous solution, which has a molar ratio (SiO₂/(A₂O+B) (A:alkali metal, B: NH₃)) of less than 4 and a silicon concentration interms of an oxide (SiO₂ concentration) of 2.0 to 12.0% by weight, bymeans of an electro-dialysis device.

In the starting alkali silicate aqueous solution, the molar ratio ofsilicon to an alkali (SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)) is lessthan 4, preferably 1.5 or more but less than 4.0, more preferably about2.8 to 3.5. The silicon concentration in terms of an oxide (SiO₂concentration) is in the range of 2.0 to 12.0% by weight, preferably 3.0to 12.0% by weight, more preferably about 4.5 to 12.0% by weight.

In the electro-dialysis device, between an anode and a cathode, cationexchange membranes 1 and anion exchange membranes 2 are alternatelydisposed and desalting zones 3 and concentration zones 4 are alternatelyarranged, as shown in FIG. 1. As the electro-dialysis device of thisconstitution, hitherto known devices are employable without anyrestriction. That is to say, also as the electrodes, ion exchangemembranes and other necessary members to constitute the electro-dialysisdevice, those publicly known are employable without any restriction. Forexample, hydrocarbon type cation exchange membranes and anion exchangemembranes, which generally have a sulfonic acid group as a cationexchange group and a quaternary ammonium group as an anion exchangegroup and include a styrene/divinylbenzene copolymer as a reinforcingsubstrate, are industrially used as the ion exchange membranes. Further,fluorine-containing ion exchange membranes using a fluorine-containingpolymer as a material are also employable.

In the electro-dialysis device, it is preferable to use alkali-resistantion exchange membranes because the starting alkali silicate aqueoussolution to be subjected to electro-dialysis is alkaline and causticalkali must be concentrated (produced).

In the electro-dialysis, a starting alkali silicate aqueous solution isfed to the desalting zone 3 of the electro-dialysis device and water ora dilute caustic alkali aqueous solution is fed to the concentrationzone 4 of the device, to perform electro-dialysis. The alkali metal ion(e.g., Na⁺) present in the desalting zone 3 passes into theconcentration zone 4 through the cation exchange membrane 1 and thehydroxide ion (OH⁻) present in the desalting zone 3 passes into theconcentration zone 4 through the anion exchange membrane 2. Thus,desalting is carried out in the desalting zone 3. On the other hand, inthe concentration zone 4, concentration of the alkali metal ion and thehydroxide ion from the desalting zone 3 is carried out to obtain acaustic alkali aqueous solution.

Although the operational conditions of the electro-dialysis device varydepending on the size of the device, the concentration of the startingalkali silicate aqueous solution, etc., the electric voltage iscontrolled so as to be constant at 0.6 V/pair, and the feed rate of thestarting alkali silicate aqueous solution to the desalting zone issuitably about 3.1 l/min. To the concentration zone, water or a dilutecaustic alkali aqueous solution is fed at a rate of about 3.1 l/min.

From the desalting zone 3, an alkali silicate aqueous solution(dealkalized solution) having been lowered in the alkali concentrationby the dealkalization is obtained.

In order to inhibit precipitation of silica solids with raising themolar ratio (SiO₂/(A₂O+B)), it is desirable to adjust the molar ratio ofthe alkali silicate aqueous solution obtained from the desalting zone 3to preferably 4.0 to 30, more preferably 9 to 26, particularlypreferably about 12 to 21.

By appropriate selection of the electro-dialysis conditions,particularly electric conductivity, the molar balance (SiO₂/(A₂O+B)) ofthe alkali silicate aqueous solution can be controlled. In general, whenthe electric conductivity is high, the value of SiO₂/(A₂O+B) tends to below, and when the electric conductivity is low, the value ofSiO₂/(A₂O+B) tends to be high.

In the first process, a starting alkali silicate aqueous solution havinga relatively high silicon concentration is used, and therefore thesilicon concentration of the resulting alkali silicate aqueous solutionbecomes preferably 6.8 to 12% by weight, more preferably 6.8 to 9% byweight, in terms of SiO₂.

In the conventional electro-dialysis of an alkali silicate aqueoussolution, a starting alkali silicate aqueous solution having arelatively low silicon concentration is used in order to inhibitclogging of ion exchange membranes and thereby perform continuousoperation. That is to say, the concentration of the starting solution isat most about 6.0% by weight in terms of SiO₂, and the concentration ofthe resulting dealkalized solution is at most about 6.2% by weight interms of SiO₂. In contrast therewith, a starting alkali silicate aqueoussolution having a relatively high silicon concentration in terms of SiO₂is used in the first process of the invention, as described above, andhence a dealkalized solution (alkali silicate aqueous solution) having ahigh silicon concentration in terms of SiO₂ can be obtained. As aresult, the high-molar ratio active alkali silicate aqueous solution ofthe invention satisfying the aforesaid properties (A) and (B),preferably further satisfying the aforesaid properties (C) to (F), canbe obtained.

Through the electro-dialysis, a caustic alkali aqueous solution isobtained from the concentration zone 4. In the caustic alkali aqueoussolution, silicic acid having passed through the ion exchange membraneduring the course of dialysis is sometimes included in a trace amount,i.e., about 0.1 to 1% by weight. Such a caustic alkali aqueous solutioncan be recycled as it is in the case where inclusion of a trace amountof silicic acid makes no matter, for example, use as an alkali sourcefor preparing an alkali silicate aqueous solution that becomes astarting material for the preparation of a silicic acid sol. Further,such a caustic alkali aqueous solution can also be used for thepreparation of alkali silicate of JIS No. 1 or No. 2, sodiummetasilicate and sodium orthosilicate each of which has a low SiO₂/A₂Oratio.

By allowing the solution of the concentration zone 4 to retain thereinduring the electro-dialysis, the alkali concentration can be lowered.

In the first process of the invention, a reverse osmosis membrane methodmay be carried out to further concentrate the dealkalized solution(alkali silicate aqueous solution) obtained from the desalting zone.

As the reverse osmosis membrane, an alkali-resistant composite membraneis preferably used because the dealkalized solution contains a traceamount of an alkali. The fractional molecular weight of the reverseosmosis membrane is in the range of preferably 100 to 20000, morepreferably 100 to 1000, particularly preferably 100 to 800. The reverseosmosis membrane method has a feature that water content is removed withsmall energy consumption without evaporating water, whereby recovery ofa valuable product (alkali silicate herein) can be performed in asolution state and concentration can be performed stably andefficiently. In the conventional method to concentrate colloidal silica,such as an evaporation concentration method wherein concentration iscarried out by raising the temperature to 100° C. that is a boilingpoint of water or a vacuum distillation method wherein concentration iscarried out by lowering the boiling point of water under vacuum,particles of the colloidal silica are allowed to grow especially underthe heating conditions. Therefore, some silicic acid anions are onlypresent on the particle surfaces, and the activity is liable to be lost.

On the other hand, an ultrafiltration method wherein water content isremoved using an organic thin film such as a film of polysulfone,polyacrylonitrile, cellulose acetate, nitrocellulose or cellulose isgenerally used from the viewpoints of energy and ease of conditioncontrol (see U.S. Pat. No. 3,969,266, U.K. Patent No. 1,148,950 andJapanese Patent Laid-Open Publication No. 15022/1983).

In the ultrafiltration method, however, there is a disadvantage thatuseful and highly active silicic acid anions which appear by theelectro-dialysis are removed.

In contrast therewith, the reverse osmosis membrane method whereinorganic thin films stable in a strong alkali aqueous solution aresterically arranged to constitute a module of excellent volumeefficiency is energy-saving, compact and easy in condition control andis capable of performing recovery of a valuable product by concentrationwithout application of heat and without modification of the product.

In the reverse osmosis, it is desirable to adjust the pressure (at theentrance of the reverse osmosis module) to preferably not more than 4.0MPa, more preferably about 3.2 to 3.8 MPa.

It is desirable to adjust the solution temperature to about 35 to 40° C.

By the use of the reverse osmosis membrane method in combination, thealkali silicate aqueous solution obtained by the electro-dialysis can befurther concentrated to a silicon concentration of preferably 3.0 to30.0% by weight, more preferably about 6.5 to 30% by weight, in terms ofSiO₂.

When the reverse osmosis membrane method is used in combination, such ahigh-silicon concentration solution as mentioned above has not to beused as the starting alkali silicate aqueous solution.

That is to say, the second process for preparing an alkali silicateaqueous solution according to the invention comprises:

dealkalizing a starting alkali silicate aqueous solution having a molarratio (SiO₂/(A₂O+B)) of less than 4 by means of an electro-dialysisdevice, and

concentrating the dealkalized solution by a reverse osmosis membranemethod.

In the starting alkali silicate aqueous solution, the molar ratio(SiO₂/(A₂O+B)) of silicon to an alkali (alkali has the same meaning aspreviously described) is less than 4, preferably 1.5 or more but lessthan 4.0, more preferably about 2.8 to 3.5. Although the siliconconcentration in terms of an oxide (SiO₂ concentration) is notspecifically restricted, it is in the range of 2.0 to 12.0% by weight,preferably 3.0 to 12.0% by weight, more preferably about 4.5 to 12.0% byweight.

The device and the conditions used for the electro-dialysis are the sameas those in the aforesaid first process.

In order to inhibit precipitation of silica solids with raising themolar ratio (SiO₂/(A₂O+B)), the molar ratio (SiO₂/(A₂O+B)) of the dilutealkali silicate aqueous solution (dealkalized solution) having beenlowered in the alkali concentration and obtained from the desalting zone3 is desirably adjusted to preferably 4.0 to 30, more preferably 9 to26, particularly preferably about 12 to 21.

In the second process, it is desirable to adjust the siliconconcentration of the dealkalized solution to preferably 3.0 to 10.0% byweight, more preferably about 4.0 to 8.0% by weight, in terms of SiO₂.

In the second process, then, the dealkalized solution obtained from thedesalting zone is concentrated by a reverse osmosis membrane method.

The reverse osmosis membrane method is carried out in the same manner aspreviously described.

By the reverse osmosis membrane method, water. content is removed fromthe dealkalized solution to concentrate the dealkalized solution (alkalisilicate solution). As a result, the high-molar ratio active alkalisilicate aqueous solution of the invention satisfying the aforesaidproperties (A) and (B), preferably further satisfying the aforesaidproperties (C) to (F), can be obtained.

The alkali concentration (in terms of an oxide) of the high-molar ratioactive alkali silicate aqueous solution obtained by the invention islowered to 0.4% by weight or less, but when needed, the alkaliconcentration can be further lowered by bringing the solution intocontact with a cation exchange resin. As the cation exchange resin,R—SO₃H type, R—COOH type and R—OH type cation exchange resins can beused without any restriction. The contact with the ion exchange resinmay be carried out after the electro-dialysis or after the reverseosmosis.

By bringing the high-molar ratio active alkali silicate aqueous solutionobtained by the electro-dialysis or by the electro-dialysis and thereverse osmosis membrane method into direct contact with the cationexchange resin, desalting proceeds in the alkali solution, whereby themolar ratio (SiO₂/(A₂O+B)) can be further enhanced. The contact with thecation exchange resin can be carried out by, for example, charging acolumn of 200 to 1000 cm³ with 240 to 530 cm³ of a cation exchangeresin, washing the cation exchange resin with water and then passing thealkali silicate aqueous solution through the column under the conditionsof pH of 5.0 to 6.0 and a flow rate of 4 to 25 ml/sec.

The high-molar ratio active alkali silicate aqueous solution of theinvention can be used for various purposes, and owing to the low alkalicontent, this solution is useful as a ground consolidation agent. Forexample, when construction work is made on the poor ground, the groundconsolidation agent is poured into the ground to impart strength anddurability to the ground. If an alkali is contained in a groundconsolidation agent, there is a fear of contamination of soil orunderground water. According to the invention, however, the alkalicontent can be extremely decreased, and hence the ground consolidationagent can be used without a fear of contamination.

Further, the high-molar ratio active alkali silicate aqueous solution ofthe invention functions also as a precursor of colloidal silica.Colloidal silica is prepared from the high-molar ratio active alkalisilicate aqueous solution of the invention in the following manner. Thealkali silicate aqueous solution of the invention is temporarily madeacidic with a mineral acid, and the salt concentration of the alkalisilicate or the like is controlled, whereby colloidal silica containingno salt so as to keep stability is prepared. Around the particles of thecolloidal silica, positive counter ions in the amounts balanced with theamounts of the surface electric charges are widely distributed toaveragely promote particle growth. According to this process, colloidalsilica of high quality can be prepared easily and at a low cost.

As described above, the high-molar ratio active alkali silicate aqueoussolution of the invention can be used in various fields where silicafine particles have been conventionally used, and this alkali silicateaqueous solution can be used for, for example, heat-resistant binder,catalyst, anti-slip agent for wrapping paper, anti-slip matting agent,various coating agents, abrasive for wafer abrasion, reinforcing agent,flocculating agent and ink-jet printing fixing agent.

According to the present invention described above, an alkali silicateaqueous solution having intermediate characteristics between water glassand colloidal silica and having a high molar ratio (SiO₂/(A₂O+B)), ahigh silicon content and a high anionic activity, a process forpreparing the solution and uses of the solution are provided.

EXAMPLES

The present invention is further described with reference to thefollowing examples, but it should be construed that the invention is inno way limited to those examples.

The electro-dialysis device and the reverse osmosis device used in theexamples are as follows.

Electro-Dialysis Device (Manufactured by Tokuyama K.K.)

Anion exchange membrane (10 membranes): AHA (trade name), available fromTokuyama K.K.

Cation exchange membrane (12 membranes): CMB (trade name), availablefrom Tokuyama K.K.

Electrode material: Ni plate

Distance between electrodes: 26.2 mm

Distance between anion exchange membrane and cation exchange membrane:0.7 mm

Area of ion exchange membrane: 2 dm²/membrane

Reverse Osmosis Device (Manufactured by Toray Engineering)

Reverse osmosis membrane: mini-spiral membrane (alkali-resistantsynthetic composite membrane, fractional molecular weight: 200, membranearea: 1.6 m², φ2.0×40 L)

High-pressure circulating pump (SUS316L/NBR)

-   -   normal: 5-12.5 L/min, 40 kgf/cm²    -   pressure-resistant: 10 L/min, 70 kgf/cm²

Spiral vessel: for φ2.0×40 L, FRP withstand pressure 70 kgf/cm²

Accumulator: bladder type, 100 cc, maximum working pressure 70 kgf/cm²

Example 1

The specific gravity and the composition of the alkali silicate aqueoussolution used as a starting material are as follows.

Specific gravity (15° C.): 1.404

SiO₂ (%): 28.12

Na₂O (%): 9.21

SiO₂/Na₂O (molar ratio): 3.15

The alkali silicate aqueous solution was diluted with water to obtain analkali silicate aqueous solution having a silicic acid concentration (interms of SiO₂) of 6% by weight.

The thus obtained starting alkali silicate aqueous solution was fed to adesalting zone of such an electro-dialysis device as previouslyspecified, and to a concentration zone of the device was fed a dilutecaustic soda solution.

Then, electro-dialysis was initiated through a constant voltageoperation at 0.6 V/pair (stack voltage: 6 V/10 pairs) and a tank(including electrode zone) voltage of 9 to 10 V. As a result, theinitial conductivity was 24 mS/cm. The electro-dialysis was carried outuntil the conductivity was lowered to less than 4.5 mS/cm from theinitiation. The average dialysis time necessary for the lowering of theconductivity to less than 4.5 mS/cm was 80 minutes. The dealkalizedsolution obtained from the desalting zone had a silica content (SiO₂) of6.4% by weight and an alkali content (Na₂O) of 0.35% by weight.

The dealkalized solution obtained from the desalting zone wastemperature-controlled to 30 to 40° C., fed to a concentration tank ofthe reverse osmosis device and concentrated under the conditions of aninlet flow rate of 10 L/min, an average pressure of 3.0 MPa and a flux(30° C.) of 35 to 28 kg /m²-hr, to obtain a high-molar ratio sodiumsilicate aqueous solution having the following composition andproperties.

(A) Molar ratio (SiO₂/Na₂O) 14.8

(B) SiO₂ concentration: 16.3% by weight

(C) ζ potential: −58.6 MV

(D) The ²⁹Si-NMR spectrum is shown in FIG. 2, in which ²⁹Si-NMR spectraof the following water glass and colloidal silica measured under thesame conditions are shown for comparison.

Water Glass (Dilute No. 3 Sodium Silicate, Available from Toso SangyoK.K.)

-   -   Specific gravity (15° C.): 1.064    -   SiO₂ (%): 5.80    -   Na₂O (%): 1.90    -   SiO₂/Na₂O (molar ratio): 3.15    -   ζ potential: −27.5 MV

Colloidal Silica (DuPont SM)

-   -   Specific gravity (15° C.): 1.216    -   SiO₂ (%): 30    -   Na₂O (%): 0.56    -   SiO₂/Na₂O (molar ratio): 55.26    -   ζ potential: −34.0 MV

The peak area at the chemical shift of −100 to −120 ppm in the ²⁹Si-NMRspectrum of the sodium silicate aqueous solution of the invention is2.28 times the peak area of the water glass and is 1.27 times the peakarea of the colloidal silica (DuPont SM).

(E) Transmittance within the wavelength region of 1000 to 200 nm: 95 to100%

The result of the ultraviolet to visible spectrophotometry is shown inFIG. 3, in which the results of the ultraviolet to visiblespectrophotometry of the colloidal silica (DuPont SM) and the followingcolloidal silica measured under the same conditions are shown forcomparison.

Colloidal Silica (DuPont HS-40)

-   -   Specific gravity (15° C.): 1.305    -   SiO₂ (%): 40    -   Na₂O (%): 0.41    -   SiO₂/Na₂O (molar ratio): 100.68    -   ζ potential: −36.7 MV

(F) Electric conductivity: 7.5 mS/cm

Methods and devices to measure the properties are as follows.

(A) Molar ratio (SiO₂/Na₂O)

SiO₂ and Na₂O were analyzed in accordance with JIS K1408, and the molarratio was calculated.

(B) SiO₂ concentration

SiO₂ was analyzed in accordance with JIS K1408.

(C) ζ potential

The ζ potential was measured by an electrophoresis light scatteringmethod using DELSA4403X manufactured by Beckmann Coalter Co.

(D) ²⁹Si-NMR measurement

The ²⁹Si-NMR was measured by means of an ALPHA-500 Model (500 MHz)manufactured by Japan Electron Optics Laboratory Co., Ltd.

(E) Transmittance

The transmittance was measured by means of an UV-550 Model manufacturedby Nippon Bunko.

(F) Electric conductivity

The electric conductivity was measured by means of an ES-12 Modelmanufactured by Horiba Seisakusho.

Example 2

The specific gravity and the composition of the alkali silicate aqueoussolution used as a starting material are as follows.

Specific gravity (15° C.): 1.404

SiO₂ (%): 28.12

Na₂O (%): 9.21

SiO₂/Na₂O (molar ratio): 3.15

The alkali silicate aqueous solution was diluted with water to obtain analkali silicate aqueous solution having a silicic acid concentration (interms of SiO₂) of 7.7% by weight.

The thus obtained starting alkali silicate aqueous solution was fed to adesalting zone of such an electro-dialysis device as previouslyspecified, and to a concentration zone of the device was fed a dilutecaustic soda solution.

Then, electro-dialysis was initiated through a constant voltageoperation at 0.6 V/pair (stack voltage: 6 V/10 pairs) and a tank(including electrode zone) voltage of 9 to 10 V. As a result, theinitial conductivity was 24 mS/cm. The electro-dialysis was carried outuntil the conductivity was lowered to less than 4.5 mS/cm from theinitiation. The average dialysis time necessary for the lowering of theconductivity to less than 4.5 mS/cm was 80 minutes. The specific gravityand the composition of the dealkalized solution obtained from thedesalting zone are as follows.

Specific gravity (15° C.): 1.060

SiO₂ (%): 8.03

Na₂O (%): 0.78

SiO₂/Na₂O (molar ratio): 10.62

The dealkalized solution obtained from the desalting zone was broughtinto contact with an ion exchange resin. That is to say, 280 cm³ of aweak acid cation exchange resin Dia Ion WK40 (available from NipponRensui K.K.) was charged in a column (φ2.8×H63 cm), washed with waterand adjusted to pH 5.79. Then, 2000 ml of the dealkalized aqueoussolution was fed at a flow rate of 12.7 ml/sec to perform desalting.

Thus, a high-molar ratio sodium silicate aqueous solution having thefollowing composition and properties was obtained.

(A) Molar ratio (SiO₂/Na₂O): 21.22

(B) SiO₂ concentration: 8.02% by weight

Example 3

The specific gravity and the composition of the alkali silicate aqueoussolution used as a starting material are as follows.

Specific gravity (15° C.): 1.404

SiO₂ (%): 28.12

Na₂O (%): 9.21

SiO₂/Na₂O (molar ratio): 3.15

The alkali silicate aqueous solution was diluted with water to obtain analkali silicate aqueous solution having a silicic acid concentration (interms of SiO₂) of 7.7% by weight.

The thus obtained starting alkali silicate aqueous solution was fed to adesalting zone of such an electro-dialysis device as previouslyspecified, and to a concentration zone of the device was fed a dilutecaustic soda solution.

Then, electro-dialysis was initiated through a constant voltageoperation at 0.6 V/pair (stack voltage: 6 V/10 pairs) and a tank(including electrode zone) voltage of 9 to 10 V. As a result, theinitial conductivity was 24 mS/cm. The electro-dialysis was carried outuntil the conductivity was lowered to less than 4.5 mS/cm from theinitiation. The average dialysis time necessary for the lowering of theconductivity to less than 4.5 mS/cm was 80 minutes. The specific gravityand the composition of the dealkalized solution obtained from thedesalting zone are as follows.

Specific gravity (15° C.): 1.060

SiO₂ (%): 8.03

Na₂O (%): 0.78

SiO₂/Na₂O (molar ratio): 10.62.

The dealkalized solution obtained-from the desalting zone wastemperature-controlled to 30 to 40° C., fed to a concentration tank ofthe reverse osmosis device and concentrated under the conditions of aninlet flow rate of 10 L/min, an average pressure of 3.0 MPa and a flux(30° C.) of 35 to 28 kg/m²-hr, to obtain a sodium silicate aqueoussolution having the following specific gravity and composition.

Specific gravity (15° C.): 1.121

SiO₂ (%): 16.3

Na₂O (%): 1.45

SiO₂/Na₂O (molar ratio): 11.60

The resulting sodium silicate aqueous solution was brought into contactwith an ion exchange resin. That is to say, 197 cm³ of a weak acidcation exchange resin Dia Ion WK40 (available from Nippon Rensui K.K.)was charged in a column (φ2.8×H63 cm), washed with water and adjusted topH 5.60. Then, 1000 ml of the alkali silicate aqueous solution was fedat a flow rate of 6.41 ml/sec to perform desalting.

Thus, a high-molar ratio sodium silicate aqueous solution having thefollowing composition and properties was obtained.

(A) molar ratio (SiO₂/Na₂O): 28.88

(B) SiO₂ concentration: 16.23% by weight

1. An alkali silicate aqueous solution having the following properties:(A) the molar ratio of silicon to an alkali (SiO₂/(A₂O+B) (A: alkalimetal, B: NH₃)) is in the range of 9 to 26, (B) the siliconconcentration in terms of an oxide (SiO₂ concentration) is in the rangeof 6.8 to 30% by weight; and (C) the ξ potential is in the range of −40MV to −80 MV.
 2. The alkali silicate aqueous solution as claimed inclaim 1, having the following properties: (D) the peak area at thechemical shift of −100 to −120 ppm in the ²⁹Si-NMR measurement is 1.35times or more of the peak area of water glass at the chemical shift of−100 to −120 ppm measured by ²⁹Si-NMR under the same conditions as inthe above measurement and is 1.20 times or more of the peak area ofcolloidal silica at the chemical shift of −100 to 120 ppm measured by²⁹Si-NMR under the same conditions as in the above measurement.
 3. Thealkali silicate aqueous solution as claimed in claim 2, having thefollowing properties: (E) the transmittance within the wavelength regionof 1000 to 200 nm measured by an absorptiometry is in the range of 90 to100%.
 4. The alkali silicate aqueous solution as claimed in claim 2,having the following properties: (F) the electric conductivity is in therange of 2.1 to 30 mS/cm.
 5. A ground consolidation agent containing thealkali silicate aqueous solution of claim 2 as a main agent.
 6. Thealkali silicate aqueous solution as claimed in claim 1, having thefollowing properties: (E) the transmittance within the wavelength regionof 1000 to 200 nm measured by an absorptiometry is in the range of 90 to100%.
 7. The alkali silicate aqueous solution as claimed in claim 6,having the following properties: (F) the electric conductivity is in therange of 2.1 to 30 mS/cm.
 8. A ground consolidation agent containing thealkali silicate aqueous solution of claim 6 as a main agent.
 9. Thealkali silicate aqueous solution as claimed in claim 1, having thefollowing properties: (F) the electric conductivity is in the range of2.1 to 30 mS/cm.
 10. A ground consolidation agent containing the alkalisilicate aqueous solution of claim 9 as a main agent.
 11. A process forpreparing an alkali silicate aqueous solution as claimed in claim 1,comprising dealkalizing a starting alkali silicate aqueous solution,which has a molar ratio (SiO₂/(A₂O+B) (A: alkali metal, B: NH₃)) of lessthan 4 and a silicon concentration in terms of an oxide (SiO₂concentration) of 2.0 to 12.0% by weight, by means of anelectro-dialysis device.
 12. The process for preparing an alkalisilicate aqueous solution as claimed in claim 11, wherein the resultingdealkalized solution is concentrated by a reverse osmosis membranemethod.
 13. The process for preparing an alkali silicate aqueoussolution as claimed in claim 12, wherein reverse osmosis is carried outusing an alkali-resistant composite membrane of a fractional molecularweight of 100 to
 20000. 14. The process for preparing an alkali silicateaqueous solution as claimed in claim 13, wherein after theelectro-dialysis and/or the reverse osmosis, the resulting alkalisilicate aqueous solution is brought into contact with a cation exchangeresin.
 15. The process for preparing an alkali silicate aqueous solutionas claimed in claim 11, wherein after the electro-dialysis, theresulting alkali silicate aqueous solution is brought into contact witha cation exchange resin.
 16. The process for preparing an alkalisilicate aqueous solution as claimed in claim 12, wherein after theelectro-dialysis and/or the reverse osmosis, the resulting alkalisilicate aqueous solution is brought into contact with a cation exchangeresin.
 17. A process for preparing an alkali silicate aqueous solutionas claimed in claim 1, comprising: dealkalizing a starting alkalisilicate aqueous solution having a molar ratio (SiO₂/(A₂O+B) (A: alkalimetal, B: NH₃)) of less than 4 by means of an electro-dialysis device,and concentrating the dealkalized solution by a reverse osmosis membranemethod.
 18. The process for preparing an alkali silicate aqueoussolution as claimed in claim 17, wherein reverse osmosis is carried outusing an alkali-resistant composite membrane of a fractional molecularweight of 100 to
 20000. 19. The process for preparing an alkali silicateaqueous solution as claimed in claim 17, wherein after theelectro-dialysis and/or the reverse osmosis, the resulting alkalisilicate aqueous solution is brought into contact with a cation exchangeresin.
 20. The alkali silicate aqueous solution as claimed in claim 1,which is obtained by the process comprising dealkalizing a startingalkali silicate aqueous solution, which has a molar ratio (SiO₂/A₂O+B)(A: alkali metal, B: NH₃)) of less than 4 and a silicon concentration interms of an oxide (SiO₂ concentration) of 2.0 to 12.0% by weight, bymeans of an electro-dialysis device.
 21. A ground consolidation agentcontaining the alkali silicate aqueous solution of claim 1 as a mainagent.